A photocurable dental restorative is a photocurable composite material comprising a polymerizable monomer, a photopolymerization initiator and a filler as chief components, is capable of imparting color tone which is the same as that of natural teeth, is easy to handle, and has been frequently used for restoring the treated teeth and has, in recent years, been used for restoring gum teeth which must bear high occlusive pressures.
As compared to natural teeth, however, the above restorative has a low mechanical strength. When, for example, the crown materials such as inlay, unlay and crown are formed of the above photocurable dental restorative to restore the teeth, the crown material is often broken when the occlusive pressure is high.
Namely, restoration of the gum teeth that bear high occlusive pressures must be accompanied by aesthetic appearance such as a small change in the color tone after restoration and excellent surface glossiness, as well as a mechanical strength large enough to withstand high occlusive pressures.
As a photocurable dental restorative featuring excellent aesthetic appearance and strength, there has heretofore been known the one highly densely blended with a mixture of an inorganic filler having a relatively large average particle size and a fine powdery inorganic filler. For example, Japanese Unexamined Patent Publication No. 26226/2000 discloses a photocurable dental composite restorative containing an inorganic filler, a polymerizable monomer and a bisacylphosphine oxide as a polymerization initiator. There has further been described that the inorganic filler is a mixture of 40 to 99% by weight of a nearly spherical inorganic particles having an average particle size in a range of not smaller than 0.1 μm but not larger than 1 μm and 60 to 1% by weight of inorganic fine particles having an average particle size of not larger than 0.1 μm, and further that the photocurable dental composite restorative features a high stability against environmental light, easy to handle and exhibits excellent mechanical strength, surface glossiness and anti-wear of opposing dentition (hereinafter, the above restorative is simply referred to as “conventional restorative I”).
It can surely be said that the above conventional restorative I is excellent in both aesthetic appearance and strength, and exhibits sufficient degree of properties in restoring, for example, front teeth. However, though the cured product of the conventional restorative I exhibits a bending strength which is as high as 200 MPa or more, its fracture toughness (KIC) which is an important property related to the breakage of the restorative is about 2.3 MPa·m1/2, which is not necessarily satisfactory for restoring gum teeth to which high occlusive forces are exerted.
In restoring the gum teeth, it can be considered to use a restorative which places more importance on the strength and fracture toughness than on the aesthetic appearance since the gum teeth are less visible than the front teeth, to use a material like the conventional restorative I having excellent aesthetic appearance for the portions exposed on the surfaces and use a photocurable dental composite restorative which places more importance on the mechanical strength than on the aesthetic appearance for the inside of the teeth.
As the photocurable dental composite restorative placing more importance on the mechanical strength than on the aesthetic appearance, there has been known the one which uses, as the filler, a mixture of irregular-shaped inorganic particles having particular particle sizes, an inorganic spherical filler having a relatively large average particle size and a fine powdery inorganic filler, the filler being highly densely blended. Concretely speaking, Japanese Unexamined Patent Publication (Kokai) No. 53519/2000 discloses a photocurable dental restorative containing a bisacylphosphine oxide as a polymerization initiator, wherein there is used, as an inorganic filler, a mixed filler comprising (a) irregular-shaped inorganic particles having an average particle size of from 1 to 9 μm but containing particles of a size of not smaller than 10 μm in an amount of not more than 3% by weight, (b) nearly spherical inorganic particles having an average particle size of not smaller than 0.1 μm but not larger than 5 μm, and (c) fine inorganic particles having an average particle size of from 0.01 to 0.1 μm, satisfying the following conditions:ma/(mb+mc)=0.2 to 3mb/(mb+mc)=0.5 to 0.99andmc/(mb+mc)=0.01 to 0.5where ma to mc are masses of the inorganic particles (a) to (c).
The cured body formed from the photocurable dental restorative (hereinafter simply referred to as “conventional restorative II”) disclosed in the prior art, exhibits surface glossiness and can be practicably used as a restorative for the gum teeth, offering bending strength and fracture toughness which are as high as 278 MPa and 3.2 MPa·mm1/2, respectively, and further featuring a high stability against environmental light and good handling property like those of the conventional restorative I.
However, the surface glossiness of the conventional restorative II is below the aesthetic level of the conventional restorative I. In restoring, for example, the front teeth maintaining good aesthetic appearance (though this method involves cumbersome operation), therefore, a crown can be fabricated by using the conventional restorative I for the portion exposed on the surface and by using the conventional restorative II that places more importance on the mechanical strength than on the aesthetic appearance for the inside of the teeth. In the dental clinic, however, there occurs a case where the surface of the conventional restorative II is not all covered with the conventional restorative I or a case where the underground is exposed due to the grinding for fine adjustment. In such cases, there arouses a problem in that a distinct difference appears in the surface glossiness between the conventional restorative I and the conventional restorative II.